Not Applicable.
Not Applicable.
Not Applicable.
1. Field of the Invention
This invention pertains generally to battery charging systems, and more particularly to charging a series of battery cells as a collection of pseudo-parallel battery segments.
2. Description of the Background Art
The use of battery chargers for periodically recharging secondary energy cells such as rechargeable batteries, secondary batteries, battery packs, storage cells, or the like is well known. Certain applications, such as battery systems within electric vehicles, employ a long string of battery cells connected in series. These xe2x80x9clong-stringxe2x80x9d battery systems are typically charged using a high-wattage charger which may be configured, for example, for constant-current/constant-voltage (CI/CV) charging.
Charging systems for long-string batteries suffer from a number of drawbacks and limitations however. In particular, a high-wattage charger system is required, and the cells of the battery are often subjected to substantial levels of both undercharging and overcharging which can lead to both lowered battery capacity and lowered battery life expectancy.
Accordingly, a need exists for a battery charger that allows for the efficient charging of batteries and battery systems which contain any number of cells. The present invention satisfies that need, as well as others, while overcoming the deficiencies of previous charging systems.
The present invention provides for pseudo-parallel charging of a series of battery cells in a battery pack. The long series of cells within the battery pack, which may comprise cells within a single battery or series of separate batteries, are divided into segments which are charged as pseudo-parallel cells so that the battery pack can be charged at greater efficiency with a lower-cost charger.
The present invention takes an approach that is contrary to what is conventionally taught in regard to charge system design, which is to charge the entire string of cells as a single indivisible unit. Charging a battery as a single unit with a single charger is simple, and the charge rate is typically set at a constant current limited by an upper charge voltage. However, since the cells and/or battery segments can differ in their capability to accept a charge, the charger is always overcharging certain portions of the battery pack while undercharging other portions. For example, it is common for weak cells, that exhibit a high charge resistance, to prematurely arrive at the charge voltage limit with the result that the remaining cells are maintained in an undercharged state. Therefore, because some of the cells are undercharged or overcharged, a long-string battery pack suffers from diminished capacity or lowered useful life.
In contrast, the present invention provides for segments in a series of battery segments to be charged in an alternating manner, referred to herein as xe2x80x9cpseudo-parallelxe2x80x9d charging. The terms xe2x80x9csegmentsxe2x80x9d or xe2x80x9cbattery segmentsxe2x80x9d generally will be used herein to refer to a portion of a series-connected set of battery cells to which conductive access is provided for the purpose of charging.
The use of pseudo-parallel charging provides a number of benefits. For example, the cost of a charging system is related to the maximum charge voltage and the amount of current to be supplied. Charging the segments of a battery individually lowers the maximum charge voltage requirement and consequently the required wattage of the charger and the associated circuit elements, such as high-current rectifiers. Furthermore, it has long been recognized that the charging efficiency of a battery charging system can be significantly enhanced by performing pulsed-current charging wherein high-current pulses are applied periodically to the battery. A description of pulsed-current charging methods may be found in an article by L. T. Lam et al. entitled xe2x80x9cPulsed-current Charging of Lead/acid Batteries - A Possible Means of Overcoming Premature Capacity Loss?xe2x80x9d from the Journal of Power Sources, issue 53 pages 215-228, published 1995, which is incorporated herein by reference. Pulsed-current charging provides high-current charge pulses separated by short relaxation intervals, on the order of 10 to 500 milliseconds, during which charging current is interrupted so as to increase the electrochemical conversion rate and efficiency of the charged battery. Pseudo-parallel charging within the present invention extends the relaxation interval of pulse-charging for a first battery segment during which a second battery segment is pulse-charged. The relaxation interval may be extended for charging batteries of more than two segments.
An object of the invention is to increase charging efficiency within a battery comprising a series of cells and/or segments.
Another object of the invention is to lower the cost of charging systems on long-string batteries.
Another object of the invention is to provide compensation of physical differences between the battery segments within a long-string battery.
Another object of the invention is to reduce the amount of overcharging or undercharging to which cells of a battery pack are exposed.
Another object of the invention is to maximize the effect of pulse-current charging without lowering charging efficiency.
Another object of the invention is to allow the use of mixed chemistries within segments of a long-string battery.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.